NO. 5 TELESCOPING OF THE CETACEAN SKULL II 



baleen whale on the one hand, is directly related to a sea-lion, while 

 a dolphin, on the other, is near kin to a fox. No more would it seem 

 reasonable to suppose that the first pair might represent two widely 

 divergent ofifshoots from one phylogenetic stock while the second 

 pair might represent similar developments from another. My object 

 is merely to show that several fundamental features of di^erence 

 between the skulls of members of the two suborders of living 

 cetaceans are, in the present absence of evidence derived from fossils, 

 most readily explained by assuming that the ancestral forms of 

 one group, at the time when telescoping was about to begin, had 

 certain critical regions of the skull built on essentially the same lines 

 as the corresponding regions in the skull of the northern furseal or 

 the northern sea-lion, while the forerunners of the other, at the cor- 

 responding period of the group's history, had them arranged essen- 

 tially as they are now seen in the fox. Terms of comparison better 

 both structurally and phylogenetically could without doubt be ob- 

 tained from the skulls of creodonts ; but these fossils cannot be dis- 

 articulated for use in preparing illustrations such as those on plates 

 2, 3 and 4, and I have therefore preferred to limit my detailed mor- 

 phological studies to recent material. Rather hasty examination of 

 the creodonts in the U. S. National Museum and the American 

 Museum of Natural History (in company with Dr. William K. 

 Gregory) has not resulted in the discovery of any genus in which 

 the furseal-mysticete conditions are clearly marked out. The most 

 that can be said is that these conditions are suggested by the structure 

 of the maxillary in Sinopa and others. On the contrary the fox- 

 odontocete type of condition is definitely present in the maxillary of 

 Hycenodon and Pterodon,<iLn a form which, so far as its actual struc- 

 ture is concerned, appears to be leading toward the anatomical features 

 present in the toothed cetacean type ; or, perhaps more properly, 

 the structure in the fossils is one which could give rise to both fox 

 and odontocete types of maxillary through two slightly different 

 courses of modification. The posterior teeth of these creodonts are 

 narrow and are situated close under the anterior base of the zygoma 

 so that there is practically no orbital plate. These features are 

 nearly the same as in Prozeuglodon (pi. 4, fig. i and pi. 5, fig. i). 

 Comparison of the skulls of Prozeuglodon and Dclphinus (pi. 5, 

 figs. I and 4) and the maxillary bone of Prozeuglodon and Grampus 

 (pi. 4, figs. I and 2) shows that the transition from the earlier to 

 the later type, so far as the particular structures under discussion 

 are concerned, would probably be mainly a matter of simple degenera- 



